Drug addiction is a chronic relapsing disorder of compulsive drug craving, seeking and use. We have identified a substantial therapeutic potential of cannabidiol (CBD) ? a non-psychoactive compound naturally occurring in the cannabis sativa plant ? against drug relapse. A 7-day treatment of CBD via a clinically relevant transdermal route reduced both cue- and stress-triggered reinstatement of cocaine as well as alcohol seeking in rats ? well- established animal models of drug relapse. Taken together with relevant reports on opioids, CBD appears to be effective against two major modes of relapse-promotion across three major classes of drugs of abuse. While the underlying neurobiological mechanism is still unclear, CBD's anti-relapse action (still evident at 138 days post-treatment) greatly outlasted its bioavailability (<18 days). It is thus likely that molecular alterations ? via long-lasting epigenetic modifications ? underlie the prolonged anti-relapse action of CBD. Previous studies have identified CBD-triggered molecular alterations in samples derived from homogenized tissues of the nucleus accumbens (NAc) ? a brain site well-implicated in drug relapse. Available evidence however indicates that cue-triggered responses ? such as drug craving and seeking ? are mediated by `cue-reactive (activated)' rather than `non-reactive' neurons in NAc (and other sites). Given that only a small subset (3-10%) of neurons in NAc (and other sites) are reactive to drug cues, molecular alterations identified in samples derived from homogenized tissues ? that contain both cue-reactive and non-reactive neurons as well as non-neural glia ? may not be functionally relevant to CBD's anti-relapse action. In contrast, long-lasting epigenetic modifications unique to neurons in which drug cue-reactivity is modulated by CBD likely mediate the prolonged anti-relapse action of CBD. Based on the scientific premise outlined above, this R21 project will test the hypothesis that ?CBD exerts its prolonged anti-relapse action via epigenetic modification of drug cue-reactive neurons?. The project will take a multidisciplinary approach incorporating (1) a well-established animal model of cocaine relapse, (2) a novel strain of transgenic rats for activity-dependent neural tagging, (3) cell type-specific molecular sampling via fluorescence activated cell sorting, and (4) next-generation sequencing for comprehensive profiling of epigenetic and transcriptional markers. The primary goal is to determine the long- lasting effects of CBD on gene expression ? that are both cell type-specific and activity-dependent (on drug cue-reactivity) ? at the level of epigenome and protein-coding transcriptome, and thereby help elucidate the molecular mechanisms functionally linked to CBD's prolonged anti-relapse action. Another primary goal is to establish the proof of concept for the functional epigenetic profiling of CBD as a potential anti-relapse medication ? a novel method that can be applied to investigate the therapeutic action of other medications. The knowledge gained through such studies then can be utilized to improve the efficacy of medical treatments via precise identification of on- and off-target effects.